[PATCH 4/6] lra-assigns.c: give up on qsort checking in assign_by_spills

[Alexander Monakov <amonakov at ispras dot ru>]

The reload_pseudo_compare_func comparator, when used from assign_by_spills,
can be non-transitive, indicating A < B < C < A if both A and C satisfy
!bitmap_bit_p (&non_reload_pseudos, rAC), but B does not.

This function was originally a proper comparator, and the problematic
clause was added to fix PR 57878:
https://gcc.gnu.org/ml/gcc-patches/2013-07/msg00732.html

That the comparator is invalid implies that that PR, if it still exists,
can reappear (but probably under more complicated circumstances).

This looks like a sensitive area, so disabling checking is the only
obvious approach.

	* lra-assigns.c (reload_pseudo_compare_func): Add a FIXME.
        (assign_by_spills): Use non-checking qsort.
---
 gcc/lra-assigns.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gcc/lra-assigns.c b/gcc/lra-assigns.c
index 2aadeef..a67d1a6 100644
--- a/gcc/lra-assigns.c
+++ b/gcc/lra-assigns.c
@@ -217,6 +217,7 @@ reload_pseudo_compare_func (const void *v1p, const void *v2p)
       /* The code below executes rarely as nregs == 1 in most cases.
 	 So we should not worry about using faster data structures to
 	 check reload pseudos.  */
+      /* FIXME this makes comparator non-transitive and thus invalid.  */
       && ! bitmap_bit_p (&non_reload_pseudos, r1)
       && ! bitmap_bit_p (&non_reload_pseudos, r2))
     return diff;
@@ -1384,7 +1385,7 @@ assign_by_spills (void)
   bitmap_ior_into (&non_reload_pseudos, &lra_optional_reload_pseudos);
   for (iter = 0; iter <= 1; iter++)
     {
-      qsort (sorted_pseudos, n, sizeof (int), reload_pseudo_compare_func);
+      qsort_nochk (sorted_pseudos, n, sizeof (int), reload_pseudo_compare_func);
       nfails = 0;
       for (i = 0; i < n; i++)
 	{
-- 
1.8.3.1